The present invention relates to an electric device or element with dielectric film and a method for manufacturing the device.
Conventionally, a bulk material, such as ceramic and single crystal material, has been used for dielectric devices; however, it is being replaced by a dielectric film for thin devices. The dielectric thin film device employs a specific structure in which a substrate supports a lower layer, a dielectric film on the lower layer and, if desired, an upper structure on the dielectric layer.
The dielectric thin film device so constructed is manufactured through various processes. Specifically, a plate made of semiconductor single-crystal is provided on its entire upper surface with the lower layer. Then, the lower layer is coated on its upper surface with the dielectric film. Further, another film for the upper structures is formed on the dielectric film, which is then patterned through a physical or chemical treatment to form the upper structures. Afterwards, the dielectric film and then the lower layer are patterned through a chemical process. The layered and patterned plate is finally separated into pieces, i.e., dielectric devices.
Another process for manufacturing the dielectric devices is disclosed in JP(A) 6-350154 in which the plate is formed on its one surface with a plurality of layers including an insulating layer, a lower electrode film, a piezoelectric film, and an upper electrode film. The plate is then etched and removed from its opposite surface to form a so-called floating structure.
The process, however, has several drawbacks. For example, the plate is different from the dielectric film in a distance of crystal lattice, a coefficient of thermal expansion, and other material features, which tends to induce a considerably high stress in the dielectric film after its formation. Also, a high temperature at the formation of the dielectric layer results in a phase transition in the dielectric layer due to a restriction force applied by the plate at cooling, which increases the internal stress in the dielectric layer.
The increased stress will lead an undesired transformation in the plate, which prevents the upper structures from being made with a great precision. The transformation also prevents the plate from being precisely fixed on an automated transporting device before its patterning. Also, forcing the plate in place will result in its undesirable damage. Besides, the transformation may change dielectric, piezoelectric, pyroelectric, and ferroelectric features of the dielectric film.
Accordingly, a primary object of the present invention is to provide a dielectric thin film device, which includes a minimal stress in a dielectric film and a reduced transformation of a substrate, providing suitable features for the dielectric film and ensuring a precise formation of upper structures.
Thus, according to the present invention, it is found that, in manufacturing a plurality of dielectric thin film devices by forming a lower layer on a substrate, coating a dielectric thin film on said lower layer, forming said dielectric thin film into a plurality of predetermined shapes, and, if desired, forming an upper structure on each of the plurality of dielectric thin films, said lower layer is formed to have a stress in a direction opposite to that of said dielectric thin film, as a result of which the stress of each component reduces.
The stress generated in the substrate is calculated by measuring its deflection and from the following equation (1):
"sgr"=(d/(D/2)2)xc3x97(E/3(1xe2x88x92xcexd))xc3x97(Ts2/Tf)xe2x80x83xe2x80x83(1)
where "sgr" is a stress in the substrate, E a Young""s modulus, Ts a thickness, xcexd a Poisson""s ratio, D a diameter, and d a deflection of the substrate.
According to the equation (1), the substrate has a deflection of 10 xcexcm or less when it has a stress of 1xc3x97108 Pa or less, which allows the upper structure to be formed with accuracy and the dielectric thin film device to be produced without being damaged in the automated transporting device.
Also, it is to be understood that, by forming a stress adjusting layer, used as protection layer for the dielectric thin film device, except said lower layer, by dividing the dielectric thin film before the formation of the upper structure, by adding an additive to the dielectric thin film, or forming a bridge structure as upper structure, the dielectric thin film can have a smaller stress.
In addition, the stress in the dielectric thin film is calculated from a peak shift of the Raman microspectroscopy, and it turns out that a stress of 1xc3x97108 Pa or less does not affect the characteristics of the dielectric thin film.
Therefore, the present invention is to provide a dielectric thin film device, in which a substrate or a lower layer formed on the substrate is coated with a dielectric thin film and then, if desired, an upper structure, characterized in that each component constituting the thin film device has a stress of 1xc3x97108 Pa or less. Alternatively, the thin film may be coated for example by PVD.
One of typical structures and methods for relaxing the stress is 1) to design the lower layer to have a stress in a direction opposite to that of the dielectric thin film. However, as an alternative of or together with the above-mentioned method, following structures and methods 2)-5) may be used.
Also, as a means of making the lower layer have a stress in a direction opposite to that of the dielectric thin film, a structure growing mechanism may be used for adjusting the stress of the lower layer for example based upon a condition under which the coating is formed.
Although, in general, a dielectric thin film often has a compressive stress, silicon nitride formed by CVD and tantalum oxide by PVD have a tensile stress.
2) A stress-adjusting layer is formed except the main components of the dielectric thin film device, the layer having a stress in a direction to relax the stress of the components.
As an example of the stress-adjusting layer, there is a thin film of such as silicon nitride or tantalum oxide.
3) At least one of La, Ca or Sr is added to the dielectric thin film. When the dielectric thin film is composed mainly of lead titanate, the addition of one of La, Ca or Sr to the dielectric thin film can relax the stress thereof. More specifically, there are a dielectric thin film that is composed mainly of lead titanate and contains 0.01-8 weight percent of lanthanum oxide, a dielectric thin film that is composed mainly of lead titanate and contains 0.01-42 weight percent of calcium titanate, and the dielectric thin film that is composed mainly of lead titanate and contains 0.01-39 weight percent of strontium titanate.
4) A mask is used on a substrate or a lower layer for divisionally forming the dielectric thin film into predetermined shapes.
5) The dielectric thin film is formed on a substrate or a lower layer, and then is divided into predetermined shapes.
A mixed solution with acid that contains a small amount of HF is used for etching the dielectric thin film, which allows a thin film device of high quality to be obtained, limiting the preferred etching of grain boundary. More specifically, the dielectric thin film is formed, and then is etched by a strong acid solution preferably with 1 volume percent or less of hydrofluoric acid into predetermined separated shapes.
Thus, the dielectric thin film operates properly without possible difficulty caused by the stress. The substrate has a deflection of 10 xcexcm or less, which allows an accurate formation of the micro upper structure. The substrate is not damaged even if fixed in the automated transporting device, which allows a yield thereof to be improved. Also, since a stress generated in the substrate is relatively small, a reliability of the upper structure is improved.